The purpose of this study was to test the three-step targeting of
tumors in mice using biotinylated antibody,
streptavidin and radiolabeled
biotin. Nude mice bearing subcutaneous LS180 human
colon cancer xenografts were intravenously administered with 200 microg of the biotinylated anti-Tn
monoclonal antibody MLS128, and 2 days later they got
intravenous injection of 50 microg of
streptavidin. They were intravenously injected 1, 4 or 7 days later with 0.5 microg of 111In-diethylenetriamine pentaacetic
acid (
DTPA)-
biotin. The
tumor uptake, determined 2 h later, was 1.4, 0.5 and 0.6% injected dose/gram of tissue (ID/g), respectively, and the blood radioactivity was 1.0, 0.2 and 0.2% ID/g, respectively. When the interval between the
streptavidin and radiolabeled
biotin injections was prolonged from 1 day to 7 days, the
tumor-to-blood ratio 2 h after injection of 111In-labeled
biotin increased from 1.5 to 4.0. Clear
tumor images were obtained as early
as 2 h after injection of radiolabeled
biotin. In conclusion, these preliminary data suggested that the three-step method using the
streptavidin-
biotin system would be applicable in an experimental mouse
tumor model and provides images of
tumors rapidly and clearly after injection of radiolabeled
biotin.